In a process of the above nature, the purge stream is taken from the loop in order to prevent a build up of inert components, eg methane and often also nitrogen, in the loop. Also, where the fresh methanol synthesis gas has an excess of one of the reactants over that required for methanol synthesis according to the equations EQU 2H.sub.2 +CO.fwdarw.CH.sub.3 OH EQU 3H.sub.2 +CO.sub.2 .fwdarw.CH.sub.3 OH+H.sub.2 O
the purge serves to remove that excess and so prevent a build-up of unreacted reactant in the synthesis loop. In many plants, hydrogen is present in an excess.
Inevitably the purge will contain, in addition to the inert components, some hydrogen and carbon oxides. While the purge can be used as fuel, eg to provide some or all of the heat required in reforming processes used to make the fresh synthesis gas, this represents a degradation or waste of some valuable components. Thus where the fresh methanol synthesis gas is hydrogen-rich, ie contains an excess of hydrogen over that required for reaction with the carbon oxides present, and as is often the case where the fresh methanol synthesis gas is derived from a feedstock such as natural gas, the purge will inevitably contain some carbon oxides and so these will be wasted.
By careful selection of the operating conditions, eg catalyst volume, synthesis temperature, purge rate, inerts content, a commercial methanol plant operating at a synthesis pressure of 100 bar abs., typically has a carbon efficiency of the order of 93-98%, ie 93-98% by weight of the carbon present in the fresh synthesis gas as carbon oxides is converted to methanol. At lower synthesis pressures the carbon efficiency is liable to be somewhat lower.
In the present invention, further methanol is produced by subjecting the purge gas to a further methanol synthesis step. Also by operating the synthesis loop at a reduced pressure an increase in throughput can be achieved without an increase in the compression power requirements. Also by effecting the further stage of methanol synthesis under conditions wherein the gas is cooled, while it is undergoing the further synthesis reaction, by indirect heat exchange with the gas being supplied to the further synthesis step, efficient operation may be achieved. As a result the carbon efficiency may be increased, and/or other advantages as will be described hereinafter may be achieved.